Do they have Excel installed?
What e-mail package are they using?
Are they trying to open the attachment within the package, or have they
saved the attachment and tried to open the saved file separately? If the
latter, is it just double-clicking on the file that doesn't work? Or have
they also tried opening Excel and using File/ Open to open the file?
What error message, or other symptoms, are they seeing? Does Excel open
without opening the file?